The Timing and Duration of the Delamerian Orogeny...

[The Journal of Geology, 2006, volume 114, p. 189–210] � 2006 by The University of Chicago. All rights reserved. 0022-1376/2006/11402-0004$15.00

189

The Timing and Duration of the Delamerian Orogeny: Correlationwith the Ross Orogen and Implications for Gondwana Assembly

John Foden, Marlina A. Elburg,1 Jon Dougherty-Page, and Andrew Burtt2

Geology and Geophysics, University of Adelaide, Adelaide, South Australia 5005, Australia(e-mail: [email protected])

A B S T R A C T

The Antarctic Ross and the Australian Delamerian orogenies are the consequence of stress transfer to the outboardtrailing edge of the newly assembled Gondwana supercontinent. This tectonic reorganization occurred in the Earlyto Middle Cambrian on completion of Pan-African deformation and subduction along the sutures between easternand western Gondwanan continental fragments. Before this, Neoproterozoic to Early Cambrian rocks in easternAustralia were formed in a passive margin and record dispersion of Rodinia with consequent opening of the proto-Pacific. Our new U-Pb and Rb-Sr geochronology shows that in the South Australian (Adelaide Fold Belt) domain ofthe Delamerian Orogen, contractional orogenesis commenced at Ma and persisted for ∼24 m.yr. until514 � 3

Ma, terminated by rapid uplift, cooling, and extension in association with posttectonic magmatism. Inte-490 � 3gration of new and published U-Pb and 40Ar-39Ar geochronology from the entire Ross-Delamerian belt shows thatalthough both the Delamerian and Ross have a synchronous late magmatic and terminal cooling history, the Rosscommenced its convergent orogenic history at ∼540 Ma. This was 25 m.yr. before Delamerian deformation began.During the Early Cambrian, eastern Australia was still in a state of extension (or transtension), with opening of theKanmantoo Basin and associated anorogenic, largely mafic magmatism. This basin received sediment from the alreadyexposed Ross Orogen to the south. The simultaneous first occurrence of strain fabrics and subduction-related mag-matism (including boninite, granite, and andesite lavas) at ∼514 Ma in New Zealand, Victoria, South Australia, NewSouth Wales, and Tasmania implies that the Delamerian Orogeny was driven by ridge-push forces transmitted onthe initiation of westward-dipping subduction. Subsequent eastward slab rollback at 490 Ma may have occurred whenthe new slab had reached the transition zone at 650-km depth, resulting in upper plate extension and anorogenicBasin and Range–style magmatism in South Australia and Tasmania (Mount Read belt). The delayed onset of sub-duction in the Australian sector of the margin implies that westward motion of the Australian portion of easternGondwana continued to be accommodated during the late Early Cambrian by subduction or deformation along eitherthe Mozambique Suture or at the northern end of the South Prince Charles Mountains–Prydz Bay suture.

Online enhancements: appendix tables.

Introduction

In order to test correlations and provide constraintson physical models of orogenesis, it is importantto determine a precise chronology of the thermal,magmatic, and structural events that define an oro-genic belt. In this article, we present new geochro-nological results from several critical sites in thesouthern part of the Adelaide Fold Belt, part of the

Manuscript received January 27, 2005; accepted October 24,2005.

1 Present address: Department of Geology and Soil Sciences,Ghent University, Krijgslaan 281 S8, 9000 Ghent, Belgium.

2 Office of Minerals and Energy Resources, Department ofPrimary Industries, Adelaide, South Australia, Australia.

Cambro-Ordovician Delamerian Orogen of south-eastern Australia (fig. 1). These new data are in-tegrated with existing age dates from the Dela-merian and Ross orogens to provide betterconstraints on the tectonic development of this tec-tonically active edge of the Gondwana super-continent.

Regional Setting of the Delamerian Orogen. TheDelamerian Orogen encompasses Precambrian andEarly Cambrian rock sequences in eastern Austra-lia that experienced Cambrian deformation andmetamorphism. It separates the Australian Pre-cambrian cratons from the younger Paleozoic to

Journal of Geology D E L A M E R I A N O R O G E N Y 191

Figure 1. A, Pre-Cretaceous configuration of the Delamerian and Ross orogens in East Gondwana. Grayregions mainly underlain by Paleoproterozoic and Archaean basement. Stippledshading p cratonic pattern p Late

Precambrian sequences deformed and metamorphosed in the Early to Middle Cambrian. CambrianBlack p Earlyrocks deformed in the Middle to Late Cambrian. Diagonal –Middle Cambrian Paleozoic outcrops notstriping p postdeformed until after the Cambrian. B, Southeastern Australia. Gray -Neoproterozoic basement. Stippledshading p pre

sediment-dominant, passive margin sequences deformed in the Delamerian. Dark shadedpattern p Neoproterozoicsequences deformed in the Delamerian Orogeny. Diagonal to Carboniferousareas p Cambrian striping p Ordovician

sequences deformed in the Paleozoic. Murray and younger undeformed marine and nonmarineBasin p Mesozoicsedimentary cover over Cambrian basement. Sydney and Triassic marine and nonmarine sedimentaryBasin p Permiancover. The Tasman Line is the eastern limit of the outcrop of known Precambrian rocks.

Mesozoic orogenic belts of eastern Australia (fig.1). It includes the South Australian Adelaide FoldBelt, the Glenelg Complex in western Victoria, theWonaminta Block in western New South Wales,and the Precambrian and Cambrian sequences inTasmania (Coney et al. 1990). The Adelaide FoldBelt stretches 1100 km from the Peake and Denisoninliers in the far north of South Australia to thewestern tip of Kangaroo Island in the south. Southfrom Australia, the Delamerian Orogen extendsinto Antarctica, where it is known as the Ross Oro-gen (Stump 1995; Goodge 1997), underlying theTransantarctic Mountains from northern VictoriaLand to the Weddell Sea (Dalziel 1991) and theninto southern Africa to form the Cape Fold Belt (fig.1).

In South Australia, Victoria, western New SouthWales and Tasmania, the Delamerian Orogen iscomposed mainly of Late Neoproterozoic (Adelaid-ean) and Early Cambrian sedimentary rocks thatwere deposited in a passive margin setting. Thesehost anorogenic rift-related mafic igneous suites(Foden et al. 2002a; Meffre et al. 2004). In SouthAustralia (and in the Glenelg region of Victoria),Early Cambrian sedimentary sequences comprisethe basal Normanville and succeeding KanmantooGroups, deposited in the Stansbury Basin (Milneset al. 1977; Preiss 1987; Haines and Flottmann1998; Haines et al. 2001). These sequences hostLate Neoproterozoic to Early Ordovician mafic ig-neous suites (Foden et al. 2002a) and Middle Cam-brian to Early Ordovician syn- to posttectonic gran-ite suites and felsic volcanics whose ages rangefrom 514 to 475 Ma (Foden et al. 2002b). In SouthAustralia, the fold belt also incorporates Paleo- toMesoproterozoic basement inliers, including thatat Mount Painter (Paul et al. 1999; Elburg et al.2003) in the north (fig. 1B). In Tasmania, the pre-Delamerian passive margin sequences are overlainby synorogenic Middle to Late Cambrian sedimen-tary sequences associated with intermediate to fel-sic volcanics (Mount Read volcanics; Crawford and

Berry 1992; Turner et al. 1998). Although mainlyobscured by younger sequences of the Cooper-Eromanga and Bowen basins, the Delamerian beltcan be traced northward to the Cape River area innorthern Queensland (fig. 1).

The Delamerian is a compressional orogeniczone with westward-verging folds and thrust faults(Offler and Fleming 1968; Fleming and White 1984;Mancktelow 1990; Jenkins and Sandiford 1992;Flottmann et al. 1994). Earliest deformation (D1)was associated with west-verging thrusts and re-sulted in low-angle S1 fabrics (Flottmann et al.1994). Up to two subsequent phases of tight toopen, upright folds developed during D2 and D3deformations, yielding S2 and S3 fabrics (Offler andFleming 1968; Mancktelow 1990). Metamorphismdeveloped at low P and high T conditions (Offlerand Fleming 1968; Dymoke and Sandiford 1992;Sandiford et al. 1992; Alias et al. 2002) with largevariation from chlorite to sillimanite grade. High-est grades are confined to restricted zones that arealso the site of syntectonic granite intrusion andstructural complexity. Deformation and metamor-phism occurred from the early Middle Cambrian( Ma; Foden et al. 1999) to the latest Cam-514 � 4brian (∼490 Ma). This activity is concurrent withsubduction-related arc volcanism 1500 km to theeast in the Takaka Terrane in New Zealand(Munker and Cooper 1995; Munker and Crawford2000).

Regional Geology and Geochronology. Knowledgeof the geochronological framework of the Paleozoicevents in the Adelaide Fold Belt has arisen ina piecemeal fashion. Compston et al. (1966) andMilnes et al. (1977) provided much of the early dataand interpretation, based mainly on their Rb-Sr andK-Ar analyses. Drexel and Preiss (1995) summa-rized these and most of the other more recent datasources. Because of the complex thermal and struc-tural history of the belt and its short absolute du-ration, many of these earlier data are insufficientlyprecise to resolve tectonic events. Because the iso-

192 J . F O D E N E T A L .

tope systems used in these earlier studies have clo-sure at temperatures below which the interior partsof the orogen cooled only during terminal orogenicstages, they date only final cooling and erosion. AU-Pb SHRIMP date ( Ma) by Cooper et al.526 � 4(1992) on zircon from a tuff layer in the basal partof the pre-Delamerian Lower Cambrian strata (theNormanville Group) has provided a critical pieceof robust evidence, placing a maximum age limiton the entire Delamerian cycle of Cambrian sedi-mentation and orogenic basin inversion. Based onnew standards, this date has recently been reas-sessed and now returns an age of Ma (Jen-522 � 2kins et al. 2002). 40Ar-39Ar data (hornblende and mi-cas) were collected and interpreted by Turner et al.(1996), and U-Pb SHRIMP and 207Pb-206Pb zirconevaporation results have been presented by Fodenet al. (1999). In addition, recent SHRIMP U-Pb ionprobe analyses of zircon were carried out to supportSouth Australian Geological Survey (PIRSA) map-ping (e.g., Fanning 1990). The present understand-ing of the temporal framework of events in the beltwas summarized in several recent studies (Hainesand Flottmann 1998; Foden et al. 1999, 2002a,2002b).

The objective of this article is to establish theprecise timing of events that constitute the Dela-merian Orogeny and to use this information todemonstrate its relationship to the Ross Orogenyin Antarctica. We utilize data from the literatureand new U-Pb and Pb-Pb isotopic measurementson samples from critical sites in the Adelaide FoldBelt where unequivocal geological relations exist(tables 1, 2; figs. 1, 2). Sites at Reedy Creek, Monar-to, and on the south coast of Kangaroo Island eachhave syn- and posttectonic igneous suites with re-lationships that permit dating of D2–D3 deforma-tion stages. Granite samples from Mannum (fig. 2)and on the Padthaway Ridge in the southeast ofSouth Australia constrain the age of posttectonicmagmatic activity. The Bungadillina monzonitefrom the Peake and Denison Ranges (Morrison andFoden 1990) in the far north of the belt was alsodated because prior work on this suite yielded agesthat (falsely) indicated significantly earlier com-mencement of orogenic activity in the north com-pared with the southern Adelaide Fold Belt.

Analytical Techniques. In this article, the newgeochronological results we report are either 207Pb-206Pb evaporation data from zircons or U-Pb anal-yses of zircons, titanites, and monazite. Samplesfor U-Pb geochronology were crushed in a stainlesssteel jaw crusher after removal of weathered rims.Crushing, sieving, Wilfley table separation, andthen heavy liquid and Franz magnetic separation

obtained nearly pure mineral separates. The finalseparates were handpicked. The bulk rock samplesfrom which the minerals were separated were an-alyzed by x-ray fluorescence (XRF) for major andtrace elements at the Department of Geology andGeophysics, Adelaide University, following proce-dures described by Elburg et al. (2001). Most ofthese were also analyzed for their Sr, Nd, and Pbisotopic compositions at Adelaide University on aFinnigan MAT 262 thermal ionization mass spec-trometer operated in static mode. These XRF andisotopic results are reported in Foden et al. (2002a,2002b). U-Pb analyses were carried out followingtechniques described by Elburg et al. (2003).

Zircons (figs. 3, 4) folded into filaments were an-alyzed by the evaporation technique (Kober 1986;Dougherty-Page and Bartlett 1999) on a FinniganMAT 262 mass spectrometer at Adelaide Univer-sity (complete data set available from the Journalof Geology office). Each of these enfolded zirconswere analyzed in a series of heating steps. Datawere collected in a static configuration, with 206Pb,207Pb, and 208Pb collected in Faraday cups and thesmaller 204Pb beam collected in a secondary elec-tron multiplier. This allows very precise measure-ment of the 204Pb/206Pb ratio and hence precise com-mon Pb corrections. Common Pb corrections weremade using the Stacey and Kramers (1975) Pbcrustal growth curve. Individual heating steps thatgave common Pb-corrected age errors (a product ofthe sum of the 2 SE of 207Pb/206Pb and the 204Pb/206Pbused to make the common Pb correction) in excessof 10 Ma (≈2%) were not included in further agecalculations.

Using methods described by Dougherty-Page andBartlett (1999) and Dougherty-Page and Foden(1996), efforts were made to maximize the numberof heating steps in order to distinguish evaporationstages with discordant mixed-age Pb. During suc-cessive heating steps of zoned zircons, it is unlikelythat different-aged components will be repeatedlymixed in identical proportions, causing apparentage variation from one heating step to the next.Therefore, when the ages given by individual heat-ing steps are plotted against their rank (fig. 5)within a sequence of ascending age, true ages willplot as near-horizontal plateaus, while mixing ar-rays will plot along lines of higher gradient. Thepoints of inflection between these arrays may beused to discriminate between those steps that aredominated by a single component and those thatcontain mixed components.

To study the causes of any observed isotopic zo-nation, zircons from each of the samples weremounted and sectioned for backscattered electron


Table 1. Detail of the Locality and Geological Relations of Dated South Australian Samples

Locality Lat. Long. SampleaSample no.

(table 2) Rock type Informal nameIgneous

formFabrichosted

Fabriccut

Delamerianevent dated

Kinchina Quarry,Murray Bridge/Monarto 35.05�S 139.10�E BC-M2 MB1-2 Granite Monarto granite Sill S2 S2 D2

Reedy Creek 34.55�S 139.13�E 779-52 01RC183 Granodiorite Reedy Creek granodiorite Pluton S3 S1/S2 1D3, !D2Reedy Creek 34.55�S 139.13�E 779-5 779-5 Quartz diorite Reedy Creek diorite Dike None S3 !D3Reedy Creek 34.55�S 139.13�E SJF-9 01RC1 Rhyolite Felsic dike Dike None S3 !D3Peake and Deni-

son Ranges 28.30�S 136.00�E PD9588 02PD-1 Monzonite Bungadillina monzonite Stock None Weak S1 D1Mannum Quarry 34.53�S 139.21�E 779-60 MAN-1 Mafic enclave Mannum granite Pluton None All !D3Padthaway 35.45�S 139.30�E PG-11 PG-11 Syenite-granite Marcollat granite Pluton None All !D3

a Foden et al. 2002b.

(BSE) imaging, which principally highlights varia-tions in Zr/Hf ratio (Hanchar and Miller 1993).

Results

Details of our new U-Pb analytical results are givenin tables 1 and 2 and figures 5 and 6, and they aresummarized together with data from literaturesources (table 3; fig. 7). Pb-Pb evaporation data onzircons are reported in appendix tables A1–A3,available in the online edition or from the Journalof Geology office. In the following sections, the de-tails of the geological context of the analyzed sam-ples together with the implications of geochrono-logical results are discussed.

Monarto Granite (Sample MB1-2). The folded, sill-like, syn-D2 Monarto granite (this is sample BC-M2 in Foden et al. 2002b) yielded zircons that arecolorless and euhedral, with well-developed crystalfaces showing fine oscillatory (magmatic) zonationoverprinted by later post-D2 recrystallization. Thedegree of recrystallization of magmatic zonation isvariable within the population, ranging from neg-ligible to complete (fig. 3). Most crystals show somerecrystallization. Twelve zircons were mounted assingle-crystal loads and were analyzed in a total of51 heating steps, of which 41 were used in furtherage determinations. These heating steps are plottedin order of ascending age in figure 5. Results arereported in appendix table A1.

As discussed by Dougherty-Page and Bartlett(1999), utilization of the Pb-Pb evaporation methodas a high-resolution geochronological tool in zirconpopulations with complex age structures requiresthe demonstration of within-error grouping of sev-eral heating step ages to form plateaus in age-ranked diagrams (fig. 5). Rather than producing asingle plateau, the Monarto granite zircon data gen-erate a continuous trend between 492 and 507 Ma,with vague plateaus at 506 and 492 Ma, giving acollective mean age of Ma. The range500 � 7/ � 8

of ages within this population (15 Ma) is muchlarger than anticipated for replicate analyses of asingle concordant age population. We note that theReedy Creek diorite (figs. 2, 4), discussed later, pro-vides a good example of the degree of reproduci-bility expected in analysis of single-stage Cambrianzircons by the evaporation technique. The steep-ening in the array at 507 Ma is taken to indicatethe presence of some older inherited zircon withan age 1510 Ma. Clearly, the recrystallization ob-served in the BSE images (fig. 3) occurred after theinitial magmatic crystallization, and the 500 �

Ma age is interpreted (fig. 5) as a mean of7/ � 8two age components, one a magmatic crystalliza-tion event and the other a younger recrystallizationevent. We took the original magmatic crystalliza-tion age of the zircons to be defined by the 10 heat-ing steps that form a subplateau at the upper endof this age range, giving an estimation for the (syn-D2) crystallization age of the Monarto granite of

Ma. The age of recrystallization is indi-506 � 1cated by the coincidence of three heating step agesat Ma at the lowermost margin of the array.492 � 6The heating step points 5–24 (fig. 5) are consideredmixtures of these two age populations.

Because the zircon Pb-Pb data suggest two phasesof zircon growth, we separated monazite from thesample and undertook conventional U-Pb analysis(table 2), reasoning that the monazite may recordthe youngest high-temperature event. We also an-alyzed feldspar from the sample in order to makemore accurate common Pb corrections. There isvery close agreement between the concordant206Pb/238U-207Pb/235U ages and both of the two-point(feldspar-monazite) 238U/204Pb-206Pb/204Pb and 235U/204Pb-207Pb/204Pb isochron ages. These are in theranges Ma and Ma, respec-497 � 3.2 494 � 11tively. The concordia age is Ma, and the493 � 1207Pb /206Pb age is Ma. (fig. 6A). These re-506 � 4sults are consistent with the Kober data and suggestthat although the Monarto granite might have anintrusive age ∼506 Ma, it was subsequently sub-

Table 2. U-Pb Isotopic Analyses and Kober Evaporation Pb-Pb Analyses

Sample Locality MineralU

(ppm)Pb

(ppm)

Ages (Ma)

%Discordance206Pb/204Pb 207Pb/204Pb 208Pb/204Pb 206Pb/238 ∗U

Error% 207Pb/235 ∗U

Error% r 206Pb/238U � 207Pb/235U � 207Pb/206Pb �

02PD1 PD Titanite 34.31 9.47 206.71 26.45 492.98 .083 .18 .658 1.27 .45 513.0 .4 513.2 .7 513.6 3.5 .0401RC183 RC Titanite 85.77 23.72 125.66 21.72 287.31 .079 1.73 .621 .55 .17 490.5 .6 490.8 2.7 492.8 14.4 .0501RC183 RC Zircon Kober 492.6 1.0779/5 RC Zircon Kober 491.0 1.001RC1 RC Titanite 125.96 22.55 422.62 38.63 608.64 .078 .52 .615 .63 .60 486.0 .5 486.5 .6 488.4 2.9 .11MB1-2 MB Monazite 6032.12 1614.41 416.00 38.43 1087.09 .080 .75 .632 .40 .97 495.2 3.7 497.3 3.2 506.4 4.3 .42MB1-2 MB Zircon Kober 506.0 1.0MB1-2 MB Zircon Kober 492.0 6.0PG11 PG Zircon 448.00 39.00 2153.00 137.11 466.45 .078 .97 .615 .55 .96 487.8 4.7 486.5 2.2 488.7 3.8 .27MAN-1 MAN Titanite 6.59 1.11 149.27 23.05 179.58 .071 .11 .568 .17 .54 444.4 .5 456.6 .8 498.1 3.9 2.6

Sample Locality MineralU

(ppm)Pb

(ppm) 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb 238U/204PbError

% 235U/204PbError

%

Isochrons238U/204Pb,206Pb/204Pb 206Pb/204Pb(I)

235U/204Pb,207Pb/204Pb 207Pb/204Pb(I)

02PD1 PD Titanite 34.31 9.47 206.71 26.45 492.98 2279.401 .10 16.532 .1002PD1 PD Feldspar 1.91 17.67 19.15 15.63 39.62 .092 .29 .001 .29 509.8 � 5.5 19.14 511.6 � 1.4 15.6301RC183 RC Titanite 85.77 23.72 125.66 21.72 287.31 1362.626 .17 9.883 .1701RC183 RC Feldspar 4.11 8.49 20.33 15.71 40.39 32.378 .20 .235 .20 491.2 � 3.6 17.77 491.8 � 1.8 15.5701RC1 RC Titanite 125.96 22.55 422.62 38.63 608.64 5168.481 .13 37.485 .1301RC1 RC Feldspar 1.42 9.70 19.31 15.68 41.47 9.793 .37 .071 .37 485.0 � 10 18.54 485.7 � 1.3 15.63MB1-2 MB Monazite 6032.12 1614.41 416.00 38.43 1087.09 4985.678 .96 36.160 .96MB1-2 MB Feldspar 1.91 17.67 19.15 15.63 39.62 .092 .29 .001 .29 494 � 11 19.14 496.6 � 3.9 15.63MAN-1 MAN Titanite 6.59 1.11 149.27 23.05 179.58 1811.145 .15 13.136 .15MAN-1 MAN Feldspar 1.09 13.99 20.05 15.71 40.14 5.156 .15 .037 .15 445.5 � 4.9 19.68 452 � 18 15.68

Note. Analyses made using a Finnigan 262 multicollector thermal ionization mass spectrometer. Localities: and Denison Ranges, Creek,PD p Peake RC p Reedy MB p MurrayBridge–Monarto, Ridge, . Errors on the Pb/U ratios and on the ages are at the 1j level and account for mass spectrometry error and uncertaintyPG p Padthaway MAN p Mannumin the mass fractionation correction and a 5% error on the assumed common Pb composition.


Figure 2. Detail of the geology of the southern Adelaide Fold Belt (Delamerian Orogen) showing the locality ofdated samples.

jected to a high-T event at 493 Ma. In the quarrythat exposes the Monarto granite, very coarse-grained, undeformed quartz-feldspar-muscovite-biotite pegmatite crosscuts all other fabrics andlithologies. Intrusion of these undeformed pegma-tites clearly postdated regional cooling below400�C, and the muscovite 40Ar/39Ar age of 478 �

Ma dates their synchronous intrusion and cooling2(Burtt and Phillips 2002).

Reedy Creek. Zircons were separated from thestrongly foliated granodiorite and undeformed di-orite whose emplacement brackets the terminalDelamerian D3 deformation at Reedy Creek (table1). These were subjected to evaporation Pb-Pb anal-ysis. BSE images of zircons from the granodiorite(sample 01RC183) show fine oscillatory (magmatic)

zonation. Inherited cores were clearly present inthe population, and several of the zircons showedetched and slightly embayed crystal faces (fig. 4A),indicating a postintrusive resorption event. Over-growth rims with oscillatory zonation are evidenceof later magmatic growth during the diorite intru-sion. The age data for the granodiorite are plottedin order of ascending age in figure 5B. Twelve fil-aments were loaded with zircons: six with largesingle crystals and six with between three and sixsmaller crystals of similar size and morphology. Atotal of 58 heating steps were collected, of which43 gave age errors of !10 Ma and were used in fur-ther age calculations. The data give an age plateauwith a well-defined upper boundary at Ma495 � 2and a lower boundary of Ma. Heating steps489 � 7

196 J . F O D E N E T A L .

Figure 3. Backscattered electron images of zircons from the Monarto granite (Kinchina Quarry), Murray Bridge.The zircons show varying degrees of patchwork replacement. A, Original magmatic oscillatory zonation. B, Oscillatoryzonation overprinted within the pyramids of the crystal. C, Overprinting of oscillatory zonation from the prismmargin (note relict original zonation). D, Almost total recrystallization. Only pyramid tips show original zonation.

within the age plateau give a mean age of 493 �Ma (2j). Results are reported in appendix table1

A2.The larger zircons from the diorite (779/5) have

very similar morphologies to those from the grano-diorite (fig. 4), indicating possible inheritance, andare consistent with the observed disaggregation ofgranodiorite blocks collapsed into the intruding di-orite. To minimize the possibility of analyzing in-herited xenocrysts, only small acicular zircons (amorphology not present in the granodiorite) wereanalyzed. Individual acicular crystals did not sup-ply sufficient Pb for analysis, so several (6–10) wereloaded into each filament. Seven filaments wereanalyzed in a total of 37 heating steps. Of these, 26gave age errors of !2% (�!10 Ma) and were usedin the age calculations (fig. 5C). Analysis of the Pbevaporated from the diorite during each of the heat-ing steps give a very well-defined age plateau witha younger limit of Ma and an older limit489 � 5

at Ma. The mean value of the ages forming493 � 4this plateau is Ma (2j; fig. 5C). By selecting491 � 1a single morphological zircon type, our results havesuccessfully isolated a single phase of zircon crys-tallization for the Reedy Creek diorite. The rangewithin the age–heating step plateau (5 m.yr.) rep-resents a good example of the degree of reproduc-ibility anticipated in the analysis of single-stageCambrian zircons by the evaporation technique.However, it is clear that despite the data quality,the granodiorite and crosscutting diorite have iden-tical ages within error. This ( Ma) must also492 � 2therefore be the age of the terminal D3 Delameriandeformation at Reedy Creek and is the same as thatof the younger magmatic event affecting the Mo-narto granite south of Reedy Creek. Results are re-ported in appendix table A3.

In order to review and refine the 207Pb-206Pb zirconevaporation results, we also collected U-Pb data(table 2) from titanites from the granodiorite (sam-


Figure 4. A, Backscattered electron image of a zircon from the Reedy Creek granodiorite (01RC183; table 2). Thecrystal shows primary magmatic oscillatory zonation with some slight widening and overprinting of original growthzones. The faces of the crystal are slightly etched. B, Backscattered electron image of a zircon from the Reedy Creekdiorite (779/5) showing primary oscillatory magmatic zonation and large rounded inclusions.

ple 01RC183; table 2) and from late (post-D3) un-deformed felsic dikes (sample 01RC1; table 2) thatcut both granodiorite and diorite. In each case, wealso analyzed feldspars as a means of making ac-curate common Pb corrections. For the granodio-rite, the 206Pb/238U and 207Pb/235U ages and boththe two-point 238U/204Pb-206Pb/204Pb and 235U/204Pb-207Pb/204Pb isochron values yield ages in the range

to Ma. The concordia age490.5 � 0.6 491.8 � 1.8was Ma (fig. 6A), and 207Pb/206Pb age was490.5 � 2

Ma. These results are completely com-493 � 14patible with the evaporation results and suggest aweighted average U-Pb age of Ma.492 � 1.5

Titanite from the felsic dike (01RC1) yielded206Pb/238U and 207Pb/235U ages and two-point 238U/204Pb-206Pb/204Pb and 235U/204Pb-207Pb/204Pb isochronvalues in the range Ma to Ma.486.5 � 0.6 485 � 10The concordia age is Ma (fig. 6A), and 207Pb/486 � 2206Pb age is Ma. These ages are younger488 � 3and statistically different from the ages of thegranodiorite and diorite and consistent with the se-quence of intrusive events. They place a minimumage on the cessation of Delamerian deformation atReedy Creek of ∼486 Ma.

Mannum and Padthaway. The undeformed Man-num pluton (Turner and Foden 1996) is a high-level,potassic, rapakivi, (A-type) syeno-granite. The in-trusion has limited outcrop, but total magnetic in-tensity imagery shows that it clearly crosscuts theReedy Creek pluton and is part of an extensive se-ries of posttectonic A-type granites and lavas withassociated mafic dikes and enclaves that extend tothe southeast into Victoria (Turner et al. 1992). The

age of this pluton provides a minimum age of thetermination of the Delamerian deformation in thesoutheastern Mount Lofty ranges.

The Mannum granite (fig. 1) hosts swarms of en-claves that result from mingling of a mafic magmawith the host granite. As demonstrated by Turnerand Foden (1996), the mafic enclaves and graniteshow initial Sr-isotopic equilibrium and are prob-ably different fractionation and mingling stages ofthe same parent magma. Based on a recalculationof the whole-rock Rb-Sr isotope data published byTurner and Foden (1996) using ISOPLOT (Ludwig1999), the eight-point model 1 (see Ludwig 1999)Rb-Sr isochron is Ma. In southeast482.3 � 4.5South Australia (Padthaway Ridge; fig. 1), thesepost-Delamerian plutons are associated at the samestructural level with contemporary felsic volcanicrocks (Turner et al. 1992; Foden et al. 1990). Thisobservation and evidence for shallow intrusion ofthe Mannum granite provided by the fine grain sizeof the first-stage intrusion and miarolitic cavitiesindicate that the upper amphibolite facies Dela-merian metamorphic complexes (Alias et al. 2002)were nearly exhumed before being intruded bythese late granite complexes.

We collected conventional zircon U-Pb isotopicdata (table 2) from one of the posttectonic granites(the Marcollat granite) from Padthaway (PG11 inTurner et al. 1992; Foden et al. 2002b). This graniteyields very large euhedral zircons without cores andyields a concordant age of Ma (fig. 6D).487 � 1.2The posttectonic granite ages from both Mannumand the Padthaway Ridge are consistent with the

198 J . F O D E N E T A L .

Figure 5. Stepwise zircon evaporation 207Pb/206Pb ageversus heating step (ranked in order of age) diagrams. A,Monarto granite (MB1-2). The apparent mean crystalli-zation age is Ma representing a mixing line500 � 7/ � 8between the age of primary magmatic crystallization at

Ma and the age of recrystallization at506 � 1 492 � 6Ma. B, Reedy Creek granodiorite (01RC183), which givesa crystallization age of Ma. C, Reedy Creek492.6 � 1.1diorite (779/5), which gives a crystallization age of

Ma. Solid symbols are the heating step plateau491 � 1data used in age calculations. Each point is the analysisof a single heating step, representing between 24 and 120determinations of each mass peak. The error bars denotethe sum of 2 SEs on the 204Pb/206Pb and 207Pb/206Pb de-terminations. A complete tabulation of these analyticaldata are available from J. Foden on request.

Reedy Creek data, indicating that the final Dela-merian deformation was older than Ma and488 � 2probably close to Ma. A large titanite crys-492 � 2tal that grew within the mafic enclaves in the Man-num granite was dated and gave an age of 449 �

Ma (two-point titanite-feldspar 238U/204Pb-206Pb/5204Pb isochron). This is similar to ages obtained for

a magmatic-hydrothermal event in the MountPainter Inlier (Elburg et al. 2003), located farthernorth in the Adelaide Fold Belt.

South Coast of Kangaroo Island. The Delamerianmagmatic evolution exposed on Kangaroo Island(figs. 1, 2) has been discussed in detail by Foden etal. (2002b). On the south coast at Vivonne Bay andat the Stun’sail Boom River mouth, migmatitecomplexes involve the development and segrega-tion of biotite granodiorite diatexite by in situ par-tial melting of Kanmantoo Group greywackes.These complexes are intruded by potassium feld-spar megacrystic S-type granites that are contem-porary with intermingled migmatite melts. Thegranite at Stun’sail Boom River yielded a SHRIMPU-Pb zircon age of Ma (Fanning 1990). The503 � 4biotite granodiorite diatexite is syntectonic; it hasa moderately developed approximately northerlydipping (30�–50�) biotite foliation. This is the youn-gest known fabric in this part of the orogenic beltand the igneous intrusive age of the megacrysticgranite puts a maximum limit of Ma on503 � 4the end of the Delamerian contraction in this partof the belt.

East of Vivonne Bay, along the south coast ofKangaroo Island, the deformed Kanmantoo Groupsequence is intruded by numerous compositedolerite–S-type rhyolite dikes (Foden et al. 2002b).These dikes are vertical and have a strongly clus-tered NW (325�) orientation. They postdate theDelamerian deformation hosted by the diatexite-migmatite-granite suites described above, intrud-ing perpendicular to the regional fabric. A SHRIMPU-Pb zircon age of Ma (Fanning 1990) of500 � 7these dikes therefore provides a minimum age ofthe cessation of Delamerian deformation in thispart of the belt, possibly between 8 and 12 m.yr.earlier than the final deformation 150 km to thenortheast at the Reedy Creek.

Discussion

Timing and Duration of Delamerian Events. The-Ma age of zircons from a tuff horizon in522 � 2

the uppermost Normanville Group (Cooper et al.1992; Jenkins et al. 2002) constrains the maximumage of the initiation of the deep-water turbidite de-position in the Kanmantoo Trough (Stansbury Ba-sin) to be mid-Early Cambrian (table 3; fig. 7). Thedating of the earliest syntectonic granite in the belt(the Rathjen Gneiss; Foden et al. 1999) at 514 � 4Ma defines the commencement of basin inversionand the onset of Delamerian contraction. The com-mencement of the Delamerian Orogeny therefore


Figure 6. U-Pb concordia diagrams showing analyses of minerals from felsic igneous rocks from the Adelaide FoldBelt (Delamerian Orogen; fig. 2). Thermal ionization mass spectrometry analytical data (of 5–10-mg multigrain pop-ulations with similar morphology and size) plotted in each of the concordia figures are reported in table 2. A, Monazitefrom the syn-Delamerian Monarto granite. B, Titanite from the late Delamerian (D3) granodiorite and from anundeformed, post-Delamerian rhyolite dike at Reedy Creek. C, Titanite from the early syn-Delamerian Bungadallinamonzonite from the Peake and Denison Ranges in the northern Delamerian Orogen (Morrison and Foden 1990). D,Zircon from the post-Delamerian Marcollat granite in the Padthaway Ridge area of SE South Australia (fig. 1).

followed a maximum of m.yr. of sediment8 � 6deposition (fig. 7). This age of initial granite for-mation is repeated at widely dispersed localitiesaround the belt, including the Peake and DenisonRanges (Bungadillina monzonite; Morrison andFoden 1990) in the far north ( Ma; table513 � 0.82), quartz porphyry intrusions in the VictorianGlenelg Inlier ( Ma; SHRIMP U-Pb on zir-514 � 3con; Ireland et al. 2002), and the Heazlewood to-nalite in western Tasmania ( Ma; SHRIMP510 � 3U-Pb on zircon; Turner et al. 1998). Our dating of

events at Reedy Creek indicates that the end ofDelamerian deformation took place at Ma492 � 2(and certainly before intrusion of the felsic dikesand sills at 487 Ma). This is the same as the age inthe Glenelg Zone where the termination of the de-formation is bracketed by the Wando tonalite( Ma; SHRIMP U-Pb on zircon) and Loftus493 � 8Creek granodiorite ( Ma; SHRIMP U-Pb on491 � 8zircon; Ireland et al. 2002; fig. 7). We also note thatthis was probably later than the end of deformationat the westernmost limit of the orogen in southern

200 J . F O D E N E T A L .

Table 3. A Summary of New and Previously Published Geochronological Results from the Delamerian Orogen inSouth Australia and the Glenelg Inlier in Western Victoria (These Data Are Plotted in Fig. 7)

Numberin figure 7 Age (Ma) Locality Rock type Reference Method

1 478 � 2 Murray Bridge Pegmatite Burtt and Phillips 2003 Ar-Ar2 482.3 � 4.5 Mannum Granite Turner and Foden 1996 Rb-Sr isochron3 484 � 7 Loftus Creek (Glenelg, Victoria) Granodiorite Ireland et al. 2002 SHRIMP4 485.7 � 3.8 Anabama Granodiorite Foden et al. 2002b Kober5 486 � .5 Reedy Creek Rhyolite dike This article 206/238U-Pb6 487.1 � 1.2 Marcollat Syenite This article U-Pb concordia7 487 � 5 Black Hill Gabbro Milnes et al. 1977 K-Ar8 491 � 1 Reedy Creek Diorite This article Kober9 491 � 8 Loftus Creek (Glenelg, Victoria) Granodiorite Ireland et al. 2002 SHRIMP10 492 � 6 Murray Bridge/Monarto Granite This article Kober11 492.6 � 1.1 Reedy Creek Granodiorite This article Kober12 493 � 7 Windsong Rhyodacite lava A. C. Burtt and C. M. Fanning,

PIRSA databaseSHRIMP

13 493 � 8 Wando (Glenelg, Victoria) Tonalite Ireland et al. 2002 SHRIMP14 495.2 � 3.7 Murray Bridge Granite This article 206/238U-Pb15 496 � 8 Arkaroola Leucogranite Elburg et al. 2003 Sm/Nd isochron16 499 � 12 Arkaroola Pegmatite Elburg et al. 2003 Rb/Sr isochron17 500 � 7 Cape Ganthaume (KI) Granite Fanning 1990 SHRIMP18 503 � 4 Stun’sail Boom River(KI) Granite A. C. Burtt and C. M. Fanning,


19 503 � 7 Rathjen Granite Foden et al. 1999 SHRIMP20 504 � 8 Stun’sail Boom River(KI) Granite A. C. Burtt and C. M. Fanning,


21 506 � 9 Arkaroola Leucogranite Elburg et al. 2003 Sm/Nd isochron22 506 � 1 Murray Bridge Granite This article Kober23 509 � 7 Cape Willoughby (KI) Granite Fanning 1990 SHRIMP24 513 � .8 Peake and Denison ranges Monzonite This article U-Pb concordia25 513.4 � 4 Tanunda Creek Granite A. C. Burtt, PIRSA database SHRIMP26 514 � 3 Quartz porphyry Granite dike Ireland et al. 2002 SHRIMP27 514 � 4 Rathjen Granite Foden et al. 1999 Kober/SHRIMP28 521 � 4 Teal Flat Volcanics Felsic volcanics Burtt et al. 2000 SHRIMP29 522 � 2 Norm Group tuff Tuff Jenkins et al. 2002 SHRIMP

Cooling ages:480 � 4 Reedy Creek Migmatite Turner et al. 1996 Ar-Ar485 � 10 Marcollat Syenite Turner et al. 1996 Ar-Ar485 � 5 Glenelg Granite Turner et al. 1996 Ar-Ar486 � 5 Reedy Creek felsic Granite Turner et al. 1996 Ar-Ar486 � 3 Taratap Granite Turner et al. 1996 Ar-Ar486 � 9 Marne mylonite Felsic mylonite Turner et al. 1996 Ar-Ar487 � 3 Willalooka Granite Turner et al. 1996 Ar-Ar487.4 � 3.5 Stun’sail Boom River (KI) Granite Foden et al. 2002b Rb-Sr490 � 4 Palmer Granite Turner et al. 1996 Ar-Ar490 � 6 Wando Granodiorite Turner et al. 1996 Ar-Ar493 � 5 Rathjen Granite Turner et al. 1996 Ar-Ar496 � 2 Normanville Sheared granite Turner et al. 1996 Ar-Ar

Note. Island. Industries and Resources, South Australia.KI p Kangaroo PIRSA p Primary

Kangaroo Island, where it must have occurred be-tween 504 Ma (deformed S-type granite at Stun’sailBoom River; fig. 2; Fanning 1990; C. M. Fanning,personal communication, 2004) and 500 Ma (un-deformed felsic dikes at Cape Gantheaume; fig. 2).It is probable that the late deformation stages arediachronous—the product of localized crustalstrain due to thermal weakening in the vicinity ofmagmatic intrusions emplaced into a prevailingstress field. New Rb-Sr and Sm-Nd isochrons ondeformed leucogranites in the Arkaroola–MountPainter area in the north also lie in the range

and Ma, respectively, indicating a506 � 9 496 � 8maximum age of cessation of Delamerian contrac-tion of Ma, based on the Rb-Sr age of a499 � 12

deformed pegmatite (Elburg et al. 2003). The De-lamerian contraction therefore continued for amaximum of m.yr. (fig. 7).24 � 5

The onset of deformation was synchronous withthe commencement of granite production. Thistiming is defined by the magmatic ages of the ear-liest syntectonic granites, such as the -Ma514 � 4Rathjen Gneiss (Foden et al. 1999). As discussed byFoden et al. (2002b), these granites are the productsof interaction between melts from decompressedasthenospheric mantle and the contemporaneousrift basin filling sediments. They range from I- toS-type. The cessation of Delamerian deformationwas marked by a change to posttectonic bimodal,substantially mantle-derived magmatism (Turner


Figure 7. Summary of the available Delamerian geochronology with 2j uncertainties, from this article and fromthe literature. The source of each individual analysis numbered 1–29 is given in table 3. These data are all magmaticemplacement ages. The shaded band (514–490 Ma) indicates the duration of Delamerian deformation. Data pointsmarked by gray unnumbered squares are cooling ages (mostly 40Ar-39Ar on biotite or hornblende), and these are listedin table 3. The South Australian sequence of magmatic events through the Delamerian period is also indicated (Fodenet al. 2002b).

et al. 1992; Turner and Foden 1996; Foden et al.2002a, 2002b), comprising A-type granite and maficintrusions. This phase of magmatism occurred inthe Early Ordovician from ∼493 to 480 Ma and im-plies a renewed influx of hot mantle beneath thebelt, possibly associated with the combined effectsof lithospheric extension and lithospheric mantledetachment.

In Tasmania, the earliest recognized Delamerianevent is the proposed obduction of ultramafic com-plexes onto Early Cambrian passive margin basaltsediment sequences (Berry and Crawford 1988;Crawford and Berry 1992). This occurred before thedeposition of the partly molassic marine turbiditesof the late Middle and Late Cambrian DundasGroup. The Ma U-Pb SHRIMP age of the510 � 3Heazlewood tonalite (Turner et al. 1998) constrainsthe timing of this Delamerian D1 event, and as inthe Adelaide Fold Belt, it occurs close to the Early-Middle Cambrian boundary. In Tasmania, the Mid-dle to Late Cambrian is characterized by wide-spread intermediate to felsic volcanism of theMount Read Volcanics and the Tyndall Group. U-Pb SHRIMP ages (Perkins and Walsh 1993) of these

range from Ma (Mount Charter rhyolite)503 � 4to Ma (Comstock Tuff). These ages are syn-494 � 4chronous with late D2 to D3 events in the AdelaideFold Belt and consistent with the limited, localizeddeformation of the Tasmanian samples.

Comparisons with the Ross Orogen in Antarctica.Our Delamerian geochronology (fig. 7) provides agood basis to make comparisons with the Ross Oro-gen. Recent studies (Pankhurst et al. 1988; Goodgeand Dallmeyer 1992, 1996; Goodge et al. 1993a,1993b; Millar and Storey 1995; Encarnacion andGrunow 1996; Goodge 1997) provide a comprehen-sive geochronological picture of the age of the RossOrogen based on good-quality, mostly U-Pb zircondates (fig. 8). Data from North and South VictoriaLand and the central Transantarctic Mountains in-dicate the onset of major intermediate to felsic vol-canism and intrusion occurred at ∼550 Ma, follow-ing on from the Neoproterozoic BeardmoreOrogeny (Rowell et al. 1993; Encarnacion and Gru-now 1996). The commencement of the Ross Orog-eny has been interpreted to be the result of majorplate reorganization and the start of subduction(Encarnacion and Grunow 1996; Goodge 1997).

202 J . F O D E N E T A L .

Figure 8. A summary of superior-quality geochrono-logical data largely based on U-Pb zircon dating from theRoss Orogen (see the text for data sources). As indicated,these are igneous or high-grade metamorphic ages. Theage limits of SE Australian Delamerian deformation fromfigure 7 are indicated. The figure clearly indicates thatthe Ross orogenic activity commenced ∼25 m.yr. beforethe Delamerian deformation started. However, the cool-ing ages from the Ross are very similar to those from theDelamerian and suggest rapid terminal exhumation at∼490 Ma.

Sedimentation occurred in localized basins and to-gether with continuation of granite intrusion, de-formation, and metamorphism through to the LateCambrian. As Myrow et al. (2002) point out, sedi-mentary basin formation and sediment generationthrough this period are probably direct conse-quences of Ross tectonism. The undeformed Gran-ite Harbour intrusives and related pegmatite andvolcanics range from 505 to 485 Ma (fig. 8). Vol-caniclastic rocks in the Thiel Mountains are alsoundeformed and have an age of 500 Ma (Pankhurstet al. 1988). These results indicate that the RossOrogen had a history of active convergent or trans-pressional tectonism (Goodge et al. 1993a, 1993b)that continued for ∼35 m.yr. starting with the firstappearance of orogenic magmatism at least as earlyas 540 Ma (Goodge et al. 1993a, 1993b). Clearly bycomparison with the Delamerian Orogen thatforms the Australian end of the belt, the Ross hadan orogenic history that started much earlier. Infact, felsic magmatism, deformation, and meta-morphism continued for 125 m.yr. in the Ross be-fore it started in the Adelaide Fold Belt, where theoldest Cambrian granite is 514 Ma (Foden et al.1999). In the Adelaide Fold Belt, much of this timeinterval was occupied by sedimentary basin for-mation with associated mantle-derived magma-tism. The Kanmantoo Group in particular is atleast 7 km thick and was deposited in only ∼8m.yr. (Haines et al. 2001), requiring a very high rateof sediment supply. Zircon provenance studies(Ireland et al. 1998) have demonstrated that theturbidite-rich Kanmantoo Group has a large pop-ulation of detrital zircons with Early Cambrian agesas well as Grenville- and Pan-African-aged popu-lations. As also observed by Wombacher andMunker (2000), these have no obvious sources tothe west of the fold belt. Haines et al. (2001) showthat basal turbidite current directions imply sedi-ment supply to the Kanmantoo Trough from thesouth. The implications of our data support thisconclusion very firmly, implying derivation of thesediments from that part of the Ross-Delamerianorogen to the south that was experiencing tectonicshortening and rapid erosion before basin inversionhad commenced at the Australian end to the north.

Goodge and Dallmeyer (1996) provide a series of40Ar-39Ar cooling ages (fig. 8) from the GeologistsRange in the central Transantarctic Mountains andfrom the Lanterman Range in Northern VictoriaLand. Their evidence suggests contrasting behaviorof the more southern part of the Ross Orogen andthe part closest to Australia. In the central Trans-antarctic Mountains, cooling that resulted from up-lift and erosion started as early as Ma and539 � 5

possibly accounts for the sediment supplied north-ward to the Kanmantoo Trough. Cooling below themuscovite closure temperature finally occurred at∼495 Ma. By contrast, Northern Victoria Land hascooling ages very similar to those in the Delamer-ian Fold Belt, with hornblende and muscovite agesin the range 488–482 Ma, implying rapid cooling(30�C/Ma), very fast uplift (1.2 mm/yr), and erosion(Goodge and Dallmeyer 1996).

The Ross Orogen extends, as the Cape Fold Belt,into southernmost Africa, where very rapid exhu-mation, cooling, and erosion rates are also reported


(Armstrong et al. 1999). The syntectonic CapeGranite there has a U-Pb SHRIMP zircon age of

Ma and 40Ar/39Ar muscovite and biotite540 � 4cooling ages of Ma.536 � 1

Pan-African. Gondwana accreted from severalmajor and probably numerous smaller continentaland oceanic arc fragments during the very looselydefined “Pan-African” orogenies (Fitzsimons 2000;Boger et al. 2001; Boger and Miller 2004; Collinsand Pisarevsky 2005). This accretion took placeover a broad period from ∼850 to 520 Ma (Dalziel1991; Pinna et al. 1993; Powell et al. 1994; Grunowet al. 1996), with the main accretion (fig. 10) oc-curring from 650 to 520 Ma (Meert 2003; Collinsand Pisarevsky 2005). Through closure and con-vergence in the Mozambique suture (fig. 9) thatpeaked at ∼620 Ma (Meert 2003), the East AfricanOrogeny assembled western Gondwana. This su-ture also extends south into Antarctica (Jacobs etal. 1998), from where more recent interpretationshave suggested that a second suture radiated fromDronning Maud Land. This passed northeastthrough the southern Prince Charles Mountains toPrydz Bay (Boger et al. 2001; Meert 2003) and sep-arated an Australian–east Antarctic continentalfragment from an Indian block (fig. 9). Boger et al.(2001) have suggested that tectonic activity fol-lowed by postcollision cooling (Mezger and Cosca1999; Rickers et al. 2000) associated with the finalstages of Gondwanan assembly continued in thisbelt from 550 to 490 Ma (Kuunga Orogeny of Meert2003). These conclusions are also supported by re-cent results from the Leeuwin Complex in south-western West Australia (fig. 10), where granulitefacies metamorphism and syntectonic granite mag-matic suites have a mean age of Ma and537 � 4extend from ∼600 Ma to Ma (Collins 2003).522 � 5

As illustrated in figure 10, the peak of the Pan-African events in Antarctica-Africa and India oc-curred between 630 and 540 Ma but had com-menced as long ago as 900 Ma (Pinna et al. 1993).A number of recent geochronological studies in theNamibian Damara Orogen in southern Africa havealso revealed Middle to Late Cambrian static meta-morphism and posttectonic felsic magmatism(510–480 Ma; Jung and Mezger 2001; Jung et al.2001; Meert 2003), the ages of which are like thoseof the late Ross and Delamerian orogens. These areimposed on a mobile belt whose main synkine-matic activity was earlier (570–550 Ma; Jung andMezger 2001) and associated with active plate con-vergence, subduction, and accretion. Interestingly,like the model we proposed earlier for the Dela-merian, the occurrence of late A-type granite in theDamara Orogen has also been ascribed to mantle

influx following lithospheric delamination (Jung etal. 1998, 2001).

The Pan-African ages (fig. 10) mostly correspondto the time of Neoproterozoic to Early Cambrianpassive marginal activity in eastern Australia. Theage cluster marking the Delamerian corresponds tothe time of rapid decline in Pan-African events andindicates a shift in the locus of strain to what hadbeen the trailing edge of the Australia-Antarcticcontinent as it became part of the new greaterGondwana supercontinent.

Convergence and the Delamerian Orogeny. An im-portant conclusion from our results is that theDelamerian and Ross orogens have common late-magmatic and cooling histories and were both theresult of stress transfer to the trailing passive mar-gin following Gondwanan supercontinent assem-bly. However, particularly toward its southern(Weddell Sea) end, the Ross commenced its sub-duction-related orogenic history at least 25 m.yr.before Delamerian deformation and subductioncommenced in Australia and during which time thebasin into which the Kanmantoo Group sedimentswere deposited continued in extension. Consistentwith this conclusion, it has been proposed that theKanmantoo Trough was developed as a jog on a left-lateral E-W transcurrent fault south of the presentAustralian southern margin and that this fault sep-arated Pacific margin sectors of Australia and Ant-arctica (Flottmann et al. 1998). The delayed onsetof subduction during the opening of the KanmantooTrough in the Delamerian zone implies that west-ward motion of the Australian portion of easternGondwana continued to be accommodated by ei-ther subduction or deformation of its western mar-gin in either the Mozambique or more probably inthe northern end of the southern Prince CharlesMountains–Prydz Bay suture during the Middle toLate Cambrian. Such a scenario is consistent withrecent geochronological results in the latter suture(Collins and Pisarevsky 2005). This conclusion isvery well supported by the -Ma age of ter-522 � 5minal mobile orogenesis in the Leeuwin Complex(fig. 10) in southwestern West Australia (Collins2003). This complex is part of the Australian cratonand predates Delamerian initiation, but it is syn-chronous with the Ross Orogeny.

Deformation in the Ross Orogen before the De-lamerian provides an obvious source for the sedi-ments in the Kanmantoo Trough, with their dis-tinctive Early Cambrian– and Grenvillean-ageddetrital zircon populations (Ireland et al. 1998).This is also the probable source for the Early toMiddle Cambrian Haupiri Group sediments in theTakaka Terrane in New Zealand. These are inter-

204 J . F O D E N E T A L .

Figure 9. The assemblage of continental fragments constituting Gondwana (after Boger and Miller 2004). PaleProterozoic to Cambrian mobile belts, diagonal to Archaean cratonicshading p Late striping p Paleoproterozoic

nuclei, checkered -age mobile belts, craton, Prince Charles Mountains,zones p Grenville K p Kalahari sPCM p SouthPrince Charles Mountains, , craton, and craton.nPCM p North M p Madagascar D p Dharwar G p Gawler

preted to have been deposited in a back arc basinto the west of the Devil River Volcanic arc, leadingto the conclusion that for this volcanic arc to re-ceive sediments from the Ross Highlands, it musthave been on the same plate and thus above a west-dipping subduction zone south of any postulatedAustralia-Antarctic fault (Wombacher and Munker2000).

By contrast with the Ross Orogen, it has beensuggested that the earliest stage of subductionalong the Australian Pacific margin was eastwardand that the Delamerian Orogeny resulted fromcollision and obduction of postulated outboard arccomplexes on the Pacific plate across the Austra-lian margin (Crawford and Berry 1992; Munker andCrawford 2000). This conclusion is mainly basedon the interpretation that the Early Cambrian Tas-manian boninite ultramafic complexes are of fore-arc origin and were emplaced as far-traveled al-lochthons by westward obduction of the Pacificplate (Berry and Crawford 1988; Crawford and Berry

1992; and see fig. 3A, 3B in Boger and Miller 2004).This model has had long acceptance but must becalled into question by recent results, includingthose reported here. Boninite-related magmas thathave clearly intruded the attenuated continentalmargin have been reported in the Victorian GlenelgZone (Kemp 2003). These are not allochthonous,and their generation would require westward sub-duction. These Victorian boninites are likely tohave the same magmatic age (∼514 Ma) as those inTasmania (Turner et al. 1998) and those of the onsetof Delamerian orogenesis in Tasmania and in SouthAustralia (Foden et al. 1999, 2002a), of the boninite/arc association in the Takaka/Devil River/MountBenson terrane in New Zealand (Munker 2000;Munker and Crawford 2000), and of earliest graniteformation in the Victorian Glenelg Zone. Becausethese ages all indicate the simultaneous appearanceof subduction-type magmatism and of contrac-tional deformation, we infer that the synchronicity


Figure 10. Age-frequency diagram depicting Delamer-ian ages (table 3) compared with those from the eventsdefining the assembly of Gondwana (from Meert 2003;largely based on U-Pb and Pb-Pb geochronological datafrom the literature). The East African Orogeny and pre-ceding arc accretion events are loosely described as thePan-African Orogeny. The “late” Pan-African KuungaOrogeny (Meert 2003) is the consequence of final assem-bly of western and eastern Gondwanan components.

of these events implies that this is the age of in-ception of subduction (fig. 11).

Because volcanism in the Takaka–Devil River arccommenced at the same time as that on the Aus-tralian continental margin in South Australia, Vic-toria, and Tasmania, it implies that the oceanicTakaka system must have been an offshore, syn-chronous (parallel) subduction system (as indicatedin figs. 9, 11) or an along-strike oceanic extensionof the eastern Australian system.

It is hard to reconcile eastward subduction at 514Ma with the presence of synorogenic granitic mag-matism in the Adelaide Fold Belt on what would(in that interpretation) have been the underthrust-ing plate. Had the period before the DelamerianOrogeny been one of Early Cambrian eastward sub-duction, then the synchronicity of this with theextension of the South Australian KanmantooTrough, with its associated E-MORB mafic mag-matism (Foden et al. 2002a), is also difficult to ex-plain. Our conclusion is that the Delamerian Orog-eny was not driven by accretion or collision with(hypothetical) offshore arcs or continental frag-

ments but is mainly the consequence of the transferof far-field (ridge-push?) stresses to the previouslyattenuated continental margin at the beginning ofsubduction. Strain from this event was heteroge-neously partitioned across the continental margin,becoming focused in thermally weakened, atten-uated crust of prior rifts (fig. 11).

We therefore conclude that the Delamerian is anorogenic event driven by the inception of subduc-tion in this part of the Gondwanan margin and isa response to the changing plate dynamics broughtabout by the completion of subduction and colli-sion to the western side that produced Gondwana(Boger et al. 2001; Boger and Miller 2004; Collinsand Pisarevsky 2005).

Termination of the Delamerian-Ross Orogeny. Anumber of lines of evidence suggest that Delamer-ian convergent deformation terminated abruptly atthe end of the Cambrian and was associated withrapid buoyant uplift and exhumation. We reach thisconclusion because 40Ar-39Ar and Rb-Sr ages of de-trital mica and 40Ar-39Ar cooling ages do not spanthe age of the orogen; instead, they cluster at theage of the termination of deformation (∼503–490Ma; Turner et al. 1996). This tectonic transitionwas marked by changes in the composition of felsicmagmas, from syntectonic I-S-type granites to bi-modal mantle-derived, magmatic suites that in-clude siliceous, potassic, posttectonic A-type gran-ites and volcanics (Turner et al. 1992;Turner 1996;Turner and Foden 1996; Foden et al. 2002b). Ter-minal Delamerian exhumation exposed and erodedthe syntectonic metamorphic and igneous com-plexes that the A-type granites and volcanics in-truded at high crustal level. 40Ar-39Ar dating (Turneret al. 1996) of detrital micas from the dominantlyOrdovician flysch (Cas 1983; Fergusson et al. 1989)in the Lachlan Fold Belt reveal that these have LateDelamerian ages of very limited range. This indi-cates that the eroding terrain was being very rapidlyexhumed through the 350�–450�C Ar–muscovite/biotite closure temperature (Turner et al. 1996).

Those ages, mostly from the literature, that re-flect closure during cooling are illustrated in figure7. These were determined mostly on biotite fromgranites and gneisses in the Adelaide Fold Belt andthe Glenelg Zone. The ages fall in a narrow rangefrom 492 to 485 Ma and are only slightly youngerthan the age of cessation of deformation reportedin this article (i.e., ∼490 Ma at Reedy Creek), whichwas determined by U-Pb methods with high closuretemperatures. Importantly, the 40Ar-39Ar age ofhornblende (Turner et al. 1996) from the MarcollatA-type granite is indistinguishable from its U-Pb

206 J . F O D E N E T A L .

Figure 11. Cartoons (horizontal dimension not to scale) depicting the stages of Gondwanan lithospheric plateassembly and the transition of the eastern Australian–Antarctic trailing margin from passive extension to convergenceand subduction during the Ross-Delamerian orogenies.

zircon age, indicating that this posttectonic igneousintrusion was emplaced into cool upper crust.

In South Australia there are no preserved proxi-mal molasse deposits that might record this ter-minal Delamerian uplift, but in Tasmania such de-posits are very widespread. These are the latestCambrian and Early Ordovician Denison Groupthat comprise terrestrial and shallow marine, con-glomerate-rich deposits and include the Jukes andOwen Conglomerates (Noll and Hall 2003). Theseare the products of apparently rapid sequential ex-humation and erosion of the Cambrian and thenPrecambrian basement. Among these basementrocks are eclogites with Rb-Sr mica ages of 485 �

Ma (Raheim and Compston 1977). This age and4a 40Ar/39Ar analysis of Ma on hornblende489 � 9from the Mount Read Volcanics (Everard and Villa1994) clearly indicate cooling due to exhumationand erosion occurred at the same time in Tasmaniaas in the Adelaide Fold Belt and in the Glenelginlier (fig. 7).

As Turner et al. (1996) noted, the coincidence of

the cooling ages from the fold belt with the ages ofdetrital micas deposited in flysch of mainly Or-dovician age to the east in Victoria seems to suggesta causal link between Delamerian uplift and ero-sion and the termination of deformation. Our re-sults strongly imply that the Delamerian Orogenyresulted from and was maintained by subduction.Evidence indicates that the fold belt reverted toextension 24 m.yr. after the inception of convergentdeformation. This is coupled with evidence thatbecause the posttectonic uplift occurs after oro-genic deformation has ceased, it is best interpretedas buoyancy-controlled exhumation. As discussedby Foden et al. (2002b), increased mafic magma-tism and the shift of initial �Nd values of post-Delamerian A-type granites indicate that this wasalso associated with new mantle influx. The 24-m.yr. duration of convergent deformation followingthe apparent start of subduction is intriguing be-cause it is the same time interval required for anewly foundering slab with a down-dip velocity of2–3 cm/yr to reach the 650-km discontinuity.


Newly established subducting slabs are demon-strated to inevitably lose most of their negativebuoyancy at this middle mantle transition zone(Ranalli et al. 2000; Kincaid and Griffiths 2004).This results in slab rollback and immediate loss oftransmission of plate-driving compressive stressesacross the plate boundary to the continental marginand would thus have terminated the DelamerianOrogeny. This in turn also severs viscous couplingof upper and lower plates in the subduction zone,eliminating flexural drag and resulting in rapid up-per plate exhumation. We suggest that the influxof hot asthenosphere mobilized by this rollbackevent would then have been the source of high-temperature posttectonic magmatism, including A-

type granites in South Australia and the MountRead Volcanics and Tyndall Group in Tasmania(fig. 11).

A C K N O W L E D G M E N T S

This article was significantly improved as a resultof constructive comments made by F. Wombacherand an anonymous reviewer. The article was alsoimproved as a result of discussions with and com-ments from numerous colleagues, including A. Col-lins, T. Crawford, N. Direen, P. Cawood, D. Glen,M. Fanning, W. Preiss, D. Taylor, and R. Caley. Wealso thank D. Bruce in the isotope laboratories atthe University of Adelaide for his assistance.

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